[ 75° ]
AN INEXPENSIVE TYPE OF SOUND-PROOF ROOM
SUITABLE FOR ZOOLOGICAL RESEARCH
BY W. H. THORPE AND R. A. HINDE
Madingley Ornithological Field Station, Department of Zoology,
University of Cambridge
{Received 22 June 1956)
INTRODUCTION
Modern developments in electronic technique have now rendered possible and
attractive the electrophysiological study of auditory organs and the critical analysis
of the sounds produced by animals. But in order to undertake such experimental
work the provision of some form of sound screening or insulation is often required.
The standard type of sound-proof room giving virtually full insulation at all audiofrequencies is of course ideal and for some types of work essential. Such rooms are,
however, very expensive to construct, since, in order to be impervious to the lower
frequencies in the auditory spectrum, there must be at least two constructional shells
sufficiently massive to be themselves non-resonant and to support between them the
considerable weight of sand which is necessary for insulation. Moreover, while a
completely an-echoic room will probably be an unnecessary or unattainable refinement, most types of experiment will necessitate that the internal walls of the room have
a high coefficient of sound absorption (i.e. be more or less non-reflective) over a wide
range of audio-frequencies. This will require, in addition, a lining of glue-foam,
'fibre-glass' board or other similar material. Specifications for the construction of
sound-proof rooms of the standard type will be found in the book by Constable &
Constable (1949) and in the publications of the Acoustical Materials Association.
Fortunately, complete sound-proofing over the full range is not necessary for
many types of zoological work. It may be sufficient to eliminate the higher frequencies only, provided the others are reduced by a known amount. This is
particularly true of work with insects, and with the songs and call notes of the
passerine birds, and with the sounds produced by the smaller mammals. The
equipment here described was developed at the Madingley Ornithological Field
Station of the Cambridge University Department of Zoology for the investigation
and analysis of the innate and learned components of the sounds produced by passerine birds (Thorpe, 1954, 1955 and 1956). For such experiments it was necessary
to be able to isolate birds, from the earliest nestling stages onwards, from the songs
and call notes of the same and related species in sound-proof rooms with a low
degree of echo. For this purpose sounds below 2 kc./sec. could be disregarded, and
it was possible to adopt a mode of construction very much cheaper than the standard
type. The details are published now in the hope that they may be of interest to
biologists working in a number of different fields.
An inexpensive type of sound-proof room
751
CONSTRUCTION
The essence of the constructional plan consists in the use of Thermacoust (woodwool slabs) as the main building material. This has the great advantage that it is
itself sufficiently strong and rigid to constitute the actual framework of building and
yet, when smoothly plastered on the outer surface, it confers a high degree of sound
insulation (from the outside), while the unplastered inner surface is reasonably
absorptive of sound. It is thus possible to avoid the expense of building the two
massive and independent double frameworks necessary to contain the two separate
layers of insulating material (sand, broken cork, dried eel-grass, etc.) required for
the standard type of sound-proof room. All that is required is to construct two
independent shells of Thermacoust, plastered on the outsides, each supported by
a separate light wooden framework.
The present paper is based on the experience gained in building two soundproof rooms. The description given below refers primarily to the second (room A)
which incorporates lessons learned in the building of the earlier one (room B).
Although, as will be seen from the tables, room B actually produces a slightly
better sound attenuation than does room A, this is because room B was at first
unsatisfactory in certain respects and was subsequently modified at considerable
extra cost. In view of this elaboration it must be regarded as the less efficient of the
two. The design of room A, if properly carried out, is both efficient and fully
adequate for its purpose. The construction of room A is illustrated in Figs. 1—3.
The principal features are as follows:
Floor. The room is built on the wooden floor of a pre-existing hut, the foundations
of which consist of four dwarf brick walls on concrete footings, running lengthwise
of the hut. The floor of the room consists of a bitumen-bonded fibre-glass mat
lying on fibre board which in its turn lies on the floor of the hut. The area of the
bitumen-bonded fibre-glass mat is slightly greater than the plan area of the outer
shell, so that it projects slightly round the edges.
Walls and roof. There are two shells, each consisting of 2 in. Thermacoust
supported on a 2 x 1 in. wooden framework. This framework lies in the 3 in. space
between the two Thermacoust shells—so that there is a 1 in. space between the
battens on the inside of the outer shell and those on the outside of the inner shell.
The nails holding the Thermacoust to the wood penetrate only a short distance into
the wood. The outside of each shell is covered in a fairly thick layer of plaster.
Doors. The doors open outwards in order to give the maximum of useful space
inside; the outer door is therefore somewhat larger than the inner. All edges of the
doors are bevelled and shut against sheet sponge rubber so that there is a tight seal
all round (Fig. 2 A).
Since the Thermacoust sheets were too narrow for the outer door to be covered
by one sheet, special precautions to prevent the plaster cracking over the joint were
necessary. The door was therefore covered with a thin metal sheet mounted over
fibre glass; this gives increased rigidity without a great increase in weight or
clumsiness.
752
W. H. THORPE AND R. A. H I N D E
Windows. Each shell has a double window—one sheet of Perspex and one 01
glass (Fig. 2B). Each sheet is supported in sponge rubber held in a wooden
frame, the latter being attached to the side of the Thermacoust (inside the Thermacoust in the inner shell, and outside in the outer).
12
,
-13
_ / _
Inchei
Fig. i. Front elevation with front of outer shell removed.
Explanation of Figs. 1-3
1, plaster skin of outer shell; 2, Thermacoust of outer shell; 3, wood frame of outer shell; 4, space
between inner and outer shells; 5, wood frame of inner shell; 6, plaster face of inner shell;
7, face of wooden frame of inner shell; 8, door; 9, bitumen-bonded mat; io, hardboard sheet;
11, ventilator shaft; 12, packing to ventilator shaft; 13, open end of ventilator shaft.
Door: 14, wooden beading frame; 15, galvanized iron sheet; 16, fibre glass; 17, plaster; 18, Thermacoust; 19, rubber facing to door frame.
Window: 20, Perspex; 21, glass; 22, rubber seating; 23, wood block; 24, Thermacoust; 25, plaster
skin.
An inexpensive type of sound-proof room
753
Inches
15 16
17
18
ESThermacoustraWoodEB R u b b e r , , Plaster
mFlbre-glass ^«Sheet metal
Inches
Fig. 2. Construction of A door and B window.
Inches
Fig. 3. Roof plan showing ventilation.
754
W. H. THORPE AND R. A. H I N D E
Ventilators. The room is ventilated by two Thermacoust ducts, the space insidebeing 3 in. square. Each duct has two right-angle bends, one in the plane of the
roof, the other at right angles to this, leading down into the interior. These ventilator shafts thus form the only points of contact between inner and outer shells
above floor level; where the shafts penetrate the shells they are insulated from them
by a packing of sponge rubber. The outer surface is plastered (Fig. 3).
Room B. This differed from room A primarily in that the inner shell was constructed of two layers of Thermacoust mounted on either side of a common wooden
frame, and in having the floor similar to the inner shell.
EFFICIENCY
The performance of the rooms was tested by generating tones of known frequency
and amplitude immediately outside and testing their intensity inside by means of a
Ribbon Microphone and a Dawe Sound Level Meter (type No. 1400 c). In order to
overcome the position errors produced by standing waves, the sound was produced
by means of a Warble Tone Generator at n o db.
Table 1. Showing efficiency of sound-proof rooms A and B expressed as attenuation
in decibels of a sound of n o db. produced by a Warble Tone Generator at
different average frequencies {expressed in cycles per second)
The internal ambient noise level due to the presence of the observer, etc., was 38 ± 1 db. This was
increased to 54 ± 1 db. by fluorescent lighting. Figures for sound level are taken relative to the
human auditory threshold reckoned as coooz dynes/cm.1 at 1 kc.
Frequency
(c./sec.)
Sound level
outside (db.)
1000
2000
4000
6000
8000
no
no
no
no
no
no
Sound level
inside (db.)
Attenuation
(db.)
55
43±2
42±2
4O±3
55
67
Room A
IOOOO
68
70
>7o
>7o
Room B
IOOO
2000
4000
no
no
no
53±i
45±i
43 ±2
57
67
The results, shown in Table 1, are self-explanatory. Since the sounds produced
by the main species being investigated contain practically no audio-frequencies
below 2000 c./sec. (2 kc), a screening which attenuates this frequency and above by
not less than 67 db. (i.e. approximately by a factor of over 3500) was felt to be
eminently satisfactory. Thus at an average of 6000 cycles (6 kc.) sound outside the
room at 110 db. was reduced inside the room to an intensity less than that produced
by the respiration, blood circulation and heart-beats of the observer.
An inexpensive type of sound-proof room
755
SUMMARY
The design of an inexpensive sound-proof room, virtually sound-proof to audiofrequencies of 2 kc. and above, is described. Since for the purposes of many
zoological experiments low frequencies can safely be disregarded, it is felt that on
account of cheapness and relative ease of construction, this type of sound-proof
room has much to recommend it.
We are greatly indebted to Mr H. R. Humphreys, of the Engineering Division of
the British Broadcasting Corporation, and to Mr N. Fleming, of the National
Physical Laboratory, for most valuable technical advice on the details of construction. We are also very grateful to the British Broadcasting Corporation for the loan
of a Warble Tone Generator and a Dawe Sound Level Meter. Mr J. A. Popple
gave indispensable advice and help in measuring the efficiency of the rooms.
REFERENCES
CONSTABLE, J. E. R. & CONSTABLE, K. M. (1949). The Principles and Practice of Sound Insulation.
Pitman: London.
THORPE, W. H. (1954). The process of song-learning in the chaffinch as measured by means of the
sound spectrograph. Nature, Land., 173, 465—9.
THORPE, W. H. (1955). Comments on The Bird Fancyers Delight; together with notes on imitation
in the subsong of the Chaffinch. Ibis, 97, 247-5 1 •
THORPE, W. H. (1956). Learning and Instinct in Animals. London: Methuen.
48
Exp. Biol. 33, 4